1. Field of the Invention
The invention relates to a method and a device for non-destructive testing of objects using ultrasonics, whereby ultrasonic pulses are introduced by an ultrasonic testing head into the respective object, and whereby ultrasonic waves are emitted by the object in response to the ultrasonic pulses introduced and are then picked up by an ultrasonic testing head and converted into electrical signals of a signal sequence.
2. Prior Art
In medical technology, an ultrasonic examination method for diagnostic purposes is known in which ultrasonic vibrations are introduced into a body to be examined that then generates ultrasonic echoes which are converted by an ultrasonic converter into electrical signals. The radiation direction of the ultrasonic vibrations is gradually changed in identical angular steps using mechanical or electronic means. An examination area in the body is limited by the two outer radiation directions. The ultrasonic echoes reflected along the paths preset by the radiation directions are converted into a sequence of measurement values that represent the reflection properties of the examination area at various measurement points that are associated with picture points of an ultrasonic image displayed on a monitor. For each picture point of the ultrasonic image, a picture value is calculated by interpolation of the measured values of those measurement points adjacent to the picture point as a result of the geometrical association between the examined area and the ultrasonic image (DE 36 40 882 A1).
In medical technology, a method is also known for delay determination of ultrasonic pulses passing through an examined area and converted by an ultrasonic converter into an electrical measurement signal. After emission of the ultrasonic pulse, the time that passes until the measurement signal has reached a preset threshold is measured. The measurement signal is amplified and fed to an analog/digital converter working with a predetermined sampling rate. The sample values of the analog/digital converter are stored. An excerpt from the measurement signal formed by a sequence of sample values is compared with an identical-length excerpt of a reference signal stored as a sequence of reference values. The excerpt of the reference signal and if necessary that of the measurement signal are displaced one node at a time until the displaced sections match up best when they are again compared with one another. The delay is determined by a digital processor, which can also have controlling function, by correction of the period thus ascertained. The sequence of reference values, which is if possible precisely proportional to the sample values of the excerpt, is found with a function which is a yardstick for the divergence of the measurement signal excerpt scaled with one factor from a correspondingly long excerpt of the reference signal (DE 32 42 284 A1).
Also known is an ultrasonic imaging device for medical diagnostic technology, having an ultrasonic converter that introduces an ultrasonic beam into a body in order to achieve sector sampling. The ultrasonic converter converts ultrasonic echo waves into echo signals that are sampled in intervals that are shorter than the division of the pixels or picture elements. The measurement values sampled along each sampling line are stored as pixel data. The data disposed at those points of two adjacent sampling lines that are identically spaced from a beam emission point are used to obtain interpolation data (DE 36 32 813 A1).
In a known method for determining the amplitude and the amplitude position of the maximum of a correlation signal, a correlator is supplied with a digitalized picture signal and a digitalized reference image. The correlator generates the correlation signal, which is passed to a memory with maximum detector that calculates the amplitude and the amplitude position. The calculated values are then interpolated in order to determine the real maximum of more closely approximated values (DE 38 12 195 A1).
Finally, it is known in measurement signal processing with high-resolution analog/digital converters to filter the analog signals with low limit frequency and to conduct the analog/digital conversion with high resolution (periodical: Technisches Messen tm, 52nd year, issue 11, 1985, pages 404-410).
It is furthermore known to interpolate discrete values of a function by polynomials (Book: A. Duschek: "Vorlesungen uber hohere Mathematik", 4th edition 1965, pages 297-301, Vol. 1).
Important characteristic quantities for an ultrasonic instrument for non-destructive materials testing in automated testing facilities are the speed and the accuracy with which crack fault signals and wall thickness signals received from fault echoes generated using the ultrasonic pulse echo method or from a rear-wall echo sequence can be picked up.
Accordingly, the quality of a ultrasonic instrument hinges on the amplitude and delay resolution achieved with a pulse sequence frequency as high as possible. By amplitude resolution we understand the accuracy with which the amplitude extreme is ascertained (positive maximum or negative minimum), or the positive or negative maximum pulse peak. The delay is the time passing between the entry of the ultrasonic pulses into the object and the reception of the ultrasonic waves leaving the object.
In an analog ultrasonic instrument, various time intervals (windows) can be selected for evaluation of the signals received from a testing head or ultrasonic sensor after amplification. For each window, a peak value memory, using which the signal extreme within the window is recorded in analog form, is necessary for amplitude determination. In conventional systems, the signal thus obtained is then digitalized for further processing in a computer.
For measurement of the delay, an additional module is necessary. With the signal peak or the flank, in the case that the threshold within a window is exceeded, a digital counter is started which is stopped with the result within a second window. To achieve the required precision, high-frequency and hence expensive digital counters are necessary here. To improve the resolution, an additional analog time measurement is often integrated using a sawtooth signal. The sawtooth amplitude value achieved at the stop event is also digitalized and converted by a calibration unit into a delay.
In a digital ultrasonic instrument, the complete signal sequence received from an ultrasonic testing head or ultrasonic sensor is digitalized immediately after amplification. The amplitude and the delay are then determined from the digital data.
The accuracy with which high-frequency pulse-like signals can be determined by digital measurement value pickup is limited by the performance data of the analog/digital converter (ADC) used. The achievable accuracy when determining the amplitude of the maximum pulse peak is determined predominantly by the sampling rate (sampling frequency) of the ADC, beside the digitalization definition (bit number). The time position of the pulse maximum--precise knowledge of which is required for determining the delays--is determined solely by the available sampling rate.
The lower the ratio of sampling frequency to signal frequency, the poorer the resolution of the amplitude and delay determination. This means that to determine short wide-band and hence high-frequency pulses, very high sampling rates are necessary.
For achieving the required accuracy, technology normally applies methods (interleaved or random sampling) in which the same event must occur several times consecutively and then be digitalized at different times. To achieve the required accuracy in the case of events that occur once only, however, demands are placed on the ADC to be used (high digitalization and high sampling rate) and on the following-on memory modules (high speed and great depth) that are at present not feasible for price reasons on account of the very high expenditure involved.
If a commercially available analog-digital converter and standard memory modules are used, the sampling density of the signals is not sufficient for precise determination of the maximum signal amplitude and its timing.